During the past few weeks, I've received more reports of severe corn rootworm injury to Bt corn following my article on this topic. Most reports are from northwestern and north-central Illinois. The affected fields share some common features--corn has been grown without rotation, and the Bt hybrids used have expressed the Cry3Bb1 protein for many successive years.

On September 13, in response to a request to confirm severe corn rootworm damage to Bt corn, I traveled to LaSalle County, in north-central Illinois. I met the producer whose field had damage suspected to be caused by corn rootworms and several seed industry representatives. I observed significant lodging in many areas of the field and began to remove random root systems for a closer look.

Many of the roots had several nodes of roots completely pruned as a result of corn rootworm feeding. After plants were dug, a few taps on the shovel quickly dislodged loose soil from the roots, revealing the extent of the injury. Once we were out of the field with roots in hand, we checked them for the expression of the Cry3Bb1 protein and found them all to be positive for this trait.

Checking rootworm Bt corn plants in LaSalle County, Illinois, for expression of the Cry3Bb1 protein, September 13.

The questions and answers below are intended to help producers make informed seed selections for next year's growing season.

How widespread is corn rootworm injury to Bt corn that expresses the Cry3Bb1 protein?

The extent of injury is difficult for me to assess adequately. Producers who are unhappy with the level of root protection afforded by these hybrids should contact their industry representatives. As more information is generated, we will collectively be able to more accurately assess the situation. At this point, I don't believe that these fields represent a "needle in the haystack," nor do I believe that control failures of Bt rootworm hybrids that express the Cry3Bb1 protein occur in most fields. I do hope that the extent of these control failures will increasingly be shared with the broader agricultural and regulatory community as we move forward this fall. With that approach, producers will be able to make more informed choices regarding Bt corn rootworm products for the 2012 growing season.

Do these reports of severe corn rootworm pruning to Bt corn (Cry3Bb1) mean that resistance has been confirmed in these fields?

No. Confirmation of resistance requires collection of adults from affected fields and conducting further detailed laboratory investigations. We should be careful not to make the leap of assuming that fields with severe rootworm injury are supporting a resistant western corn rootworm population.

Prior to this season, has significant root pruning been observed on Bt corn (Cry3Bb1)?

Yes. We have observed significant pruning on Bt corn (Cry3Bb1) in our corn rootworm efficacy trials soon after these Bt hybrids were commercialized. In our experiments, we use a trap crop (late-planted corn interplanted with pumpkins) to increase the density of larvae so that we adequately challenge soil insecticides, insecticidal seed treatments, and Bt corn rootworm hybrids. In 2005 we reported on the variability in root protection of Bt corn rootworm hybrids that expressed the Cry3Bb1 protein in our On Target report (Adobe PDF). So the recent reports of significant damage in producers' fields should be taken seriously, but be aware that severe pruning to Bt corn has been observed previously by us, by other investigators, and by producers.

Remember, corn rootworm Bt products offer only a low to moderate dose of Cry proteins. Nonetheless, these proteins are designed to protect root systems against damaging levels of root injury, and producers have just cause for concern when excessive pruning results in lodging and significant yield loss.

Has root pruning been observed on corn rootworm Bt hybrids that express other Cry proteins?

Bottom line: the collective experience with corn rootworm injury and Bt hybrids has not been as clean or smooth as with the Bt lineup of products targeted at the lepidopteran complex (e.g., European corn borer) in the north-central region of the United States. As I learn more about the most recent reports of corn rootworm injury to Bt hybrids this season, I will be sure to share the information with readers of the Bulletin.

If I've had root protection problems with a certain type of Bt hybrid this season, how should I proceed for 2012?

If you experienced significant lodging and root pruning with a certain type of Bt hybrid in 2011, then I urge you to look at the recommendations I outlined in issue 20. I look forward to discussing this issue at fall and winter University of Illinois Extension meetings.--Mike Gray